The invention relates to a method and a system which to a listener can create an artificial auditory impression corresponding to a certain space. Particularly the invention relates to the transfer of such an auditory impression in a system which in digital form transfers, processes and/or compresses information to be presented to a user.
A virtual acoustic environment refers to an auditory impression, with the aid of which a person listening to an electrically reproduced sound can imagine himself to be in a certain space. A simple means to create a virtual acoustic environment is to add reverberation, whereby the listener gets an impression of a space. Complicated virtual acoustic environments often try to imitate a certain real space, whereby it is often called the auralisation of said space. This concept is described for instance in the article M. Kleiner, B.-I. Dalenbxc3xa4ck, P. Svensson: xe2x80x9cAuralizationxe2x80x94An Overviewxe2x80x9d, 1993, J. Audio Eng. Soc., Vol. 41, No. 11, pp. 861-875. In a natural way the auralisation can be combined with the creation of a virtual visual environment, whereby a user provided with suitable display devices and speakers or earphones can observe a desired real or imagined space, and even xe2x80x9cmovexe2x80x9d in said space, whereby his audio-visual impression is different depending on which point in said environment he selects to be his observation point.
The creation of a virtual acoustic environment is divided into three factors, which are the modelling of the sound source, the modelling of the space, and the modelling of the listener. The present invention relates particularly to the modelling of the space, whereby an aim is to create an idea about how the sound propagates, how it is reflected and attenuated in said space, and to convey this idea in an electrical form to be used by the listener. Known methods for modelling the acoustics of a space are the so called ray-tracing and the image source method. In the former method the sound generated by the sound source is divided into a three-dimensional bundle comprising xe2x80x9csound raysxe2x80x9d propagating in a substantially rectilinear manner, and then a calculation is made about how each ray propagates in the space being processed. The auditory impression obtained by the listener is generated by adding the sound represented by those rays which, during a certain period and via a certain maximum number of reflections, arrive at the observation point chosen by the listener. In the image source method a plurality of virtual image sources are generated for the original sound source, whereby these virtual sources are mirror images of the sound source regarding the examined reflecting surfaces: behind each examined reflecting surface there is placed one image source having a direct distance to the observation point which equals the distance between the original sound source and the observation point as measured via the reflection. Further, the sound from the image source arrives at the observation point from the same direction as the real reflected sound. The auditory impression is obtained by adding the sounds generated by the image sources.
The prior art methods present a very heavy calculation load. If we assume that the virtual environment is transferred to the user for instance by a radio broadcasting or via a data network, then the user""s receiver should continuously trace even as much as tens of thousands of sound rays or add the sound generated by thousands of image sources. Moreover, the basis of the calculation changes always when the user decides to change the position of the observation point. With present devices and prior art methods it is practically impossible to transfer the auralised sound environment.
The object of the present invention is to present a method and a system with which a virtual acoustic environment can be transferred to a user at a reasonable calculation load.
The objects of the invention are attained by dividing the environment to be modelled into sections, for which there are created parametrisized reflections and/or absorption models as well as transmission models, and by treating mainly the parameters of the model in the data transmission.
The method according to the invention is characterised in that there the surfaces are represented by parametrisized filters.
The invention also relates to a system, which is characterised in that it comprises means for forming a filter bank comprising parametrisized filters for the modelling of the surfaces.
According to the invention the acoustic characteristics of a space can be modelled in a manner, the principle of which is as such known from the visual modelling of surfaces. Here a surface means quite generally an object of the examined space, whereby the object""s characteristics are relatively homogenous regarding the model created for the space. For each examined surface there are defined a plurality of coefficients (in addition to its visual characteristics, if the model contains visual characteristics) which represent the acoustic characteristics of the surface, whereby such coefficients are for instance the reflection coefficient, the absorption coefficient and the transmission coefficient. More generally we may state that a certain parametrisized transfer function is defined for the surface. In the model to be created of the space said surface is represented by a filter, which realises said transfer function. When a sound from the sound source is used as an input to the system, the response generated by the transfer function represents the sound when it has hit said surface. The acoustic model of the space is formed by a plurality of filters, of which each represents a certain surface in the space.
If the design of the filter representing the acoustic characteristics of the surface, and the parametrisized transfer function realised by the filter are known, then for the representation of a certain surface it is sufficient to give the transfer function parameters characterising said surface. In a system intended to transfer a virtual environment as a data stream there is a receiver and/or a reproducing device, into the memory of which there is stored the type or types of the filter and of the transfer function used by the system. The device gets the data stream functioning as its input data, for instance by receiving it by a radio or a television receiver, by downloading it from a data network, such as the Internet network, or by reading it locally from a recording means. At the start of the operation the device gets in the data stream those parameters which are used for modelling the surfaces within the virtual environment to be created. With the aid of these data and the stored filter types and transfer function types the device creates a filter bank which corresponds to the acoustic characteristics of the virtual environment to be created. During operation the device gets within the data stream a sound, which it must reproduce to the user, whereby it supplies the sound into the filter bank which it has created, and as a result it gets the processed sound, and the user listening to this sound perceives an impression of the desired virtual environment.
The required amount of transmitted data can be further reduced by forming a data-base comprising certain standard surfaces and being stored in the memory of the receiver/reproduction device. The database contains parameters, with which it is possible to describe the standard surfaces defined by the database. If the virtual environment to be created comprises only standard surfaces, then only the identifiers of the standard surfaces in the database have to be transmitted within the data stream, whereby the parameters of the transfer functions corresponding to these identifiers can be read from the database and it will not be necessary to transfer them separately to the receiver/reproduction device. The database can also contain information about such complex filter types and/or transfer functions, which are no similar to those filter types and transfer functions which are generally used in the system, and which would consume unreasonably much of the system""s data transmission capacity if they should be transmitted with the data stream when required.